1. Field of the Invention
This invention relates to a method for reactivating an iridium-containing catalyst that has been at least partially deactivated by the deposit of carbonaceous residues thereon and by the agglomeration of the iridium.
2. Discussion of Related Art
For many years, the petroleum industry has used reforming, or hydroforming, processes for upgrading virgin or cracked naphthas to produce high octane products. In reforming, a dual-functinal catalyst (i.e., a catalyst having an acid function and a hydrogenation-dehydrogenation function) is employed. A metal component, or components, is substantially atomically dispersed upon the surface of a porous, inorganic oxide support (notably alumina) to provide the necessary hydrogenation-dehydrogenation function. Platinum catalysts, particularly metal promoted platinum catalysts, are currently employed. Reforming is defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by (1) dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics, (2) dehydrogenation of paraffins to yield olefins, (3) dehydrocyclization of paraffins and olefins to yield aromatics, (4) isomerization of n-paraffins, (5) isomerization of alkylcycloparaffins to yieldcyclohexanes, (6) isomerization of substituted aromatics, and (7) hydrocracking of paraffins to produce gas and coke, the latter being deposited on the catalyst.
The activity of the catalyst gradually declines in reforming due to the build-up of carbonaceous deposits, or coke, on the catalyst which physically blocks the catalytically active metal and acidic sites. During operation, the temperature of the process is gradually raised to compensate for the activity loss. Eventually, however, economics requires reactivating the catalyst. Consequently, in all processes of this type, the catalyst must be periodically regenerated by burning-off the coke at controlled conditions.
In the regeneration of unpromoted platinum catalysts, reactivating the catalyst has required catalyst regeneration, or burning of the coke from the catalyst, followed by redispersing the agglomerated metal by halogen treatment. For example, coke can be readily burned from a coked platinum catalyst by contact with an atmosphere of oxygen, or oxygen and chlorine gas, at flame front temperatures of about 540.degree. C., and oxygen concentrations up to about 6 volume percent. The agglomerated metal can then be readily redispersed to return the catalyst activity to essentially that of a fresh catalyst. Thus, the agglomerated platinum metal is redispersed to a fine state of dispersion, with relative ease, by treatment with chloride or other halogen-containing reagent, generally used in admixture with oxygen at elevated temperatures to increase the rate of redispersion.
However, this approach is not suitable for iridium containing, or iridium promoted, platinum catalysts. At such conditions the iridium component is severely agglomerated and the catalyst easily damaged. Iridium agglomeration reduces the metal surface area of the catalyst, thereby reducing catalyst activity and activity maintenance (i.e., cycle length). By iridium agglomeration is meant the percentage of total iridium atoms on the catalyst that is in clusters of 50 .ANG., or greater, as measured by x-ray diffraction. Once agglomerated, iridium is very difficult to redisperse, and the agglomerated iridium causes carbon or coke to be retained on the catalyst. Increasing the chloride level of an iridium-containing catalyst has been found to suppress agglomeration of the iridium, but the chloride combines with the more reactive carbon to form a flameproof species of coke. Hence, the reactivation of iridium-containing catalysts presents a more complex problem than presented by the earlier non-iridium promoted platinum catalysts.
Techniques useful for the redispersion of platinum are not directly applicable for the redispersion of iridium, or iridium in admixture with other metal hydrogenation-dehydrogenation components. Unlike platinum, large iridium and iridium oxide crystallites are formed under the conditions at which coke is readily removed and the platinum redispersed. Once formed, the iridium and iridium oxide crystallites are not readily redispersed to their original high surface area state bya simple halogen treatment immediately following the burning operation. Recently, faced with an acute need, techniques have been developed by virtue of which iridium, or iridium in admixture with other metal hydrogenation-dehydrogenation components, can be redispersed to a high surface area state. Patents exemplifying the state-of-the-art of regenerating and redispersing the iridium component of iridium-containing catalysts are U.S. Pat. Nos. 3,904,510; 3,937,660; 3,939,061; 3,939,062; 3,941,682; 3,941,716; 3,943,052; 3,981,823; 3,998,755; 4,018,670; 4,046,673; 4,148,749; 4,172,817; 4,277,369; 4,359,400; 4,444,895; 4,444,896; 4,444,897; 4,447,551; 4,467,045; 4,472,514; 4,472,515; 4,473,656; 4,480,046; 4,514,284; and 4,517,076. Foreign patents of interest are GB 2,091,577A; DDR 150,986; DDR 149,846; DDR 151,556; and European patent application No. 00936321. U.S. Pat. Nos. 4,444,895; 4,444,896; and 4,480,046 (the disclosures of which are incorporated herein by reference) are of particular interest because each involves the redispersion of iridium using a gas at a low flow rate.
However, all of these references reactivate iridium-containing catalysts under flow conditions. None teach or suggest the non-flow reactivation procedure described hereinafter.